Review

A Brief Overview of Recent Progress in Porous Silica asCatalyst Supports

Preeti S. Shinde 1, Pradnya S. Suryawanshi 1, Kanchan K. Patil 1, Vedika M. Belekar 1, Sandeep A. Sankpal 2,Sagar D. Delekar 2 and Sushilkumar A. Jadhav 1,*

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Citation: Shinde, P.S.; Suryawanshi,

P.S.; Patil, K.K.; Belekar, V.M.;

Sankpal, S.A.; Delekar, S.D.; Jadhav,

S.A. A Brief Overview of Recent

Progress in Porous Silica as Catalyst

Supports. J. Compos. Sci. 2021, 5, 75.

https://doi.org/10.3390/jcs5030075

Academic Editors: Francesco

Tornabene and Salete Balula

Received: 7 February 2021

Accepted: 3 March 2021

Published: 6 March 2021

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1 School of Nanoscience and Technology, Shivaji University Kolhapur, Vidyanagar, Kolhapur,Maharashtra 416004, India; preetishinde114@gmail.com (P.S.S.); pradnyasuryawanshi0607@gmail.com (P.S.S.);kanchanpatil66891@gmail.com (K.K.P.); vedikab1899@gmail.com (V.M.B.)

2 Department of Chemistry, Shivaji University Kolhapur, Vidyanagar, Kolhapur, Maharashtra 416004, India;sandeeporg1@gmail.com (S.A.S.); sddelekar7@gmail.com (S.D.D.)

* Correspondence: sushil.unige@gmail.com; Tel.: +91-0231-260-9000

Abstract: Porous silica particles have shown applications in various technological fields includingtheir use as catalyst supports in heterogeneous catalysis. The mesoporous silica particles haveordered porosity, high surface area, and good chemical stability. These interesting structural ortextural properties make porous silica an attractive material for use as catalyst supports in variousheterogeneous catalysis reactions. The colloidal nature of the porous silica particles is highly useful incatalytic applications as it guarantees better mass transfer properties and uniform distribution of thevarious metal or metal oxide nanocatalysts in solution. The catalysts show high activity, low degreeof metal leaching, and ease in recycling when supported or immobilized on porous silica-basedmaterials. In this overview, we have pointed out the importance of porous silica as catalyst supports.A variety of chemical reactions catalyzed by different catalysts loaded or embedded in porous silicasupports are studied. The latest reports from the literature about the use of porous silica-basedmaterials as catalyst supports are listed and analyzed. The new and continued trends are discussedwith examples.

Keywords: porous silica; catalyst; heterogeneous catalysis; catalyst support; stability

1. Introduction

Porous silicas are chemically and thermally stable materials with uniform pore size,pore distribution, high surface area, and high adsorption capacity [1–3]. The size andshape of the porous silica particles as well as the structure of pores on them can be tunedby controlling synthetic parameters like temperature, reaction time, and the amount ofsilicates/silica source; adjusting the surfactant concentration; changing the calcinationconditions; etc. [4,5]. The pore size and its uniformity contribute to the strength of poroussilica material [6]. A huge number of reports appear in the literature on silica and poroussilica materials and their applications [7–13]. This shows their versatility and use in varioustechnological or industrial applications. In particular, mesoporous silica nanoparticlesare useful in several fields of application, such as environmental, biomedical, energy, andas catalyst supports [14]. They are also used in drug delivery [15], vaccine development,biomass conversion, and as catalyst or catalyst supports [16]. Due to the ordered porosityand unique features, they also act as highly efficient nano adsorbents for the adsorptionremoval of various toxic pollutants [17,18]. These extended applications of porous silicaparticles are due to the ease of their functionalization of both the internal and externalsurfaces of their pores with various organic functional groups [19]. The porous silicaparticles can also be used as a strong support matrix in catalytic applications [20].

The first report about the synthesis of ordered mesoporous silica material was inearly 1990 [21]. Recently, there are several modified and new synthetic techniques for

J. Compos. Sci. 2021, 5, 75. https://doi.org/10.3390/jcs5030075 https://www.mdpi.com/journal/jcs

J. Compos. Sci. 2021, 5, 75 2 of 17

the synthesis of porous silica particles. The newly invented techniques provide someadvantages over the old methods. They offer control over synthesis conditions duringnucleation and growth process [22]. Due to this, it is possible to produce silica nanoparticleswith pore diameters ranging from microporous (below 2 nm) and mesoporous (2–50 nm)to macroporous (above 50 nm) [23]. The most common types of silica materials in themesoporous pore size range are Mobil Crystalline Materials-41 (MCM-41) [24]; Santa Bar-bara Amorphous (SBA-15) [25,26] with hexagonal pore structure; and other types such asHiroshima Mesoporous Material (HMM-33), Technical Delft University (TUD-1), foldedsheets mesoporous materials (FSM-16), SBA-16, MCM-48, SBA-11, SBA-12, SBA-16, KIT-5,etc. Table 1 enlists the most common types of porous silica particles with their characteristicfeatures and properties. Figure 1 shows the structures of different types of mesoporoussilica nanoparticles. Due to the excellent chemical stability and the possibility of incorpora-tion of various nanomaterials (catalysts), porous silica materials have received increasedattention as catalyst supports. Among the various types of mesoporous silica materialsmentioned above, MCM-50, SBA-11, and SBA-12 are reported as excellent adsorbent andcatalytic supports [27].

Table 1. List of different types of mesoporous silica nanoparticles (MSNs) and their characteristicproperties [27].

MSN Family MSN Type Pore Symmetry Pore Size (nm) Pore Volume (cm3/g)

M41SMCM-41 2D hexagonal 1.5–8 >1.0MCM-48 3D cubic 2–5 >1.0MCM-50 Lamellar 2–5 >1.0

SBA

SBA-11 3D cubic 2.1–3.6 0.68SBA-12 3D hexagonal 3.1 0.83SBA-15 2D hexagonal 6–0 1.17SBA-16 Cubic 5–15 0.91

KIT KIT-5 Cubic 9.3 0.45

COK COK-12 Hexagonal 5.8 0.45

FDU FDU-12 3D Cubic 10–26 0.66

J. Compos. Sci. 2021, 5, 75 2 of 19

The first report about the synthesis of ordered mesoporous silica material was in

early 1990 [21]. Recently, there are several modified and new synthetic techniques for the

synthesis of porous silica particles. The newly invented techniques provide some ad-

vantages over the old methods. They offer control over synthesis conditions during nu-

cleation and growth process [22]. Due to this, it is possible to produce silica nanoparticles

with pore diameters ranging from microporous (below 2 nm) and mesoporous (2–50 nm)

to macroporous (above 50 nm) [23]. The most common types of silica materials in the mes-

oporous pore size range are Mobil Crystalline Materials-41 (MCM-41) [24]; Santa Barbara

Amorphous (SBA-15) [25,26] with hexagonal pore structure; and other types such as Hi-

roshima Mesoporous Material (HMM-33), Technical Delft University (TUD-1), folded

sheets mesoporous materials (FSM-16), SBA-16, MCM-48, SBA-11, SBA-12, SBA-16, KIT-

5, etc. Table 1 enlists the most common types of porous silica particles with their charac-

teristic features and properties. Figure 1 shows the structures of different types of meso-

porous silica nanoparticles. Due to the excellent chemical stability and the possibility of

incorporation of various nanomaterials (catalysts), porous silica materials have received

increased attention as catalyst supports. Among the various types of mesoporous silica

materials mentioned above, MCM-50, SBA-11, and SBA-12 are reported as excellent ad-

sorbent and catalytic supports [27].

Figure 1. Different types of mesoporous silica nanoparticles. Reproduced with permission from

the authors of [27].

Table 1. List of different types of mesoporous silica nanoparticles (MSNs) and their characteristic properties [27].

MSN Family MSN Type Pore Symmetry Pore Size (nm) Pore Volume (cm3/g)

M41S

MCM-41 2D hexagonal 1.5–8 >1.0

MCM-48 3D cubic 2–5 >1.0

MCM-50 Lamellar 2–5 >1.0

SBA

SBA-11 3D cubic 2.1–3.6 0.68

SBA-12 3D hexagonal 3.1 0.83

SBA-15 2D hexagonal 6–0 1.17

SBA-16 Cubic 5–15 0.91

KIT KIT-5 Cubic 9.3 0.45

COK COK-12 Hexagonal 5.8 0.45

FDU FDU-12 3D Cubic 10–26 0.66

Figure 1. Different types of mesoporous silica nanoparticles. Reproduced with permission from theauthors of [27].

J. Compos. Sci. 2021, 5, 75 3 of 17

The solid or porous silica materials are most commonly synthesized by sol-gel andhydrothermal processes. These methods include the use of reagents such as tetraethoxysi-lane (TEOS) as a silica source, cetyltrimethylammonium bromide (CTAB) as a templatingagent, trimethyl benzene (TMB) as a modulator to tune the pore diameter, and alcoholas a solvent in combination with water [28,29]. The most established way to synthe-size the catalyst-immobilized porous silica is using catalyst material as a base in a sol–gel process that will result in densely structured particles impregnated on or inside thehighly branched silica network [30]. The composition and pore structure of the catalyst-immobilized porous silica materials can be studied with various characterization techniquessuch as infrared spectroscopy (IR), X-ray diffraction (XRD), thermogravimetric analysis(TGA), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), and nuclear magnetic resonance (NMR) [31,32]. These tech-niques can be used to conform the formation of siloxane network, porosity, formation, andincorporation of the catalyst particles in the silica matrix. Porous silicas are good catalystsupports because of their inertness, multi-functionalities, and stability in almost all solventsand high catalytic selectivity. The first report of using mesoporous silica in a polymersynthesis catalysis was reported by Aida’s group in Japan, which opened a new route foradvanced solid supported catalysis [33]. Their ability to catalyze (by virtue of the catalystloaded) any reaction of alkylation, arylation, or vinylation of various alkenes in organiccatalysis makes them an attractive material [34]. Their ability to readily separate from theproduct after reaction completion is another characteristic feature of porous silicas [35].Porous silica-based materials have also been investigated as supports for enzymes such ascytochrome C (MW-12k) and showed that the immobilization of enzymes on inorganic ma-terial like porous silica is very useful in practical applications [36]. This is a classic exampleof the potential of porous silicas to improve the stability of biomolecules or an enzymeunder extreme conditions. In this brief review, we have presented basic information aboutporous silica particles and highlighted their use as catalyst supports. The requirements ofgood catalyst supports are discussed, and the very latest reports from the literature aboutthe use of porous silica-based materials as catalyst supports are enlisted with analysis.

MCM: Mobil Crystalline Materials; SBA: Santa Barbara Amorphous; KIT: KoreaAdvanced Institute of Science and Technology, COK: Centre for Research Chemistry andCatalysis Mobil Crystalline, FDU: Fairleigh Dickinson University.

2. Catalyst Support Properties and Requirements

Catalyst supports are important to support solid catalysts as they increase the effi-ciency of the supported metals or metal oxides by acting as a catalytically active center.The support can be chemically inert or it may interact with the active component (actualcatalyst). Note that the interactions of the reactants in solid, liquid, or in gaseous formswith the support material must be non-destructive. The interactions of the support materialwith the active catalyst thereby affect the catalyst activity and selectivity. The supportmaterial may not contribute directly to the catalytic reaction process but may contributeindirectly by adsorbing the reactants near the embedded catalysts. The materials used ascatalyst supports must show chemical stability, high surface area, as well as capability ofdispersing metal or metal oxide particles highly over their surface. This is very importantwhen expensive metals, such as gold, silver, platinum, ruthenium, palladium, etc., are usedas the catalysts. Nanoparticles of noble metals are prepared to obtain the catalyst withhigh surface area and the supports must expose the right sides or maximum surface of thenanomaterials for the chemical reaction to occur. Supports give the catalyst its physicalform, texture, mechanical resistance, and certain activity particularly for bifunctional cata-lysts. The surface chemical (functional groups) and physical properties of surfaces affectthe performance of the supported metals. By keeping in mind these requirements, variousoxides and carbon compounds are being used as catalyst support materials. Among allmaterials, silica (SiO2) acts as an excellent catalyst support material due to its outstandingchemical and physical properties. The porosity plays an important role in increasing the

J. Compos. Sci. 2021, 5, 75 4 of 17

efficiency of catalyst supports. The shape as well as the size of pores of the support havean important effect on the activity and stability of embedded catalysts. Metal nanoparticlessupported on porous silica-based supports exhibit higher catalytic activity arising from thehigher accessibility of the active sites. A variety of porous silica particles are available ascatalyst supports. Table 2 lists the main types of porous silica materials used as catalystsupports and their characteristic properties. The chemical inertness and high stabilitymake these materials ideal catalyst supports. Figure 2 shows a representative exampleof how nickel nanoparticles are loaded or trapped inside different types of mesoporoussilica-based catalyst supports. The different arrangements of the embedded catalyst insidethe porous silica are clearly visible.

Table 2. Comparison of the properties of various porous silica particles.

Full NameSanta BarbaraAmorphous

Type 15

Santa BarbaraAmorphous

Type 16

MobilComposition ofMatter No. 41

MobilComposition ofMatter No. 48

HexagonalMesoporous

Silica

Short name SBA-15 SBA-16 MCM-41 MCM-48 HMS

Structure directingagent

Pluronic 123(non-ionic)

Pluronic F127(non-ionic) CTAB (cationic) CTAB (cationic) Amines (non-ionic)

pH at synthesis Acidic(pH ~ 1)

Acidic(pH ~ 1)

Basic(pH ~ 11–13)

Basic(pH ~ 11–13)

Basic(pH ~ 9)

Features

Hexagonal pores,2D array, p6mm

symmetry,channels

interconnected bysmall micropores

3D cubicarrangementconnected by

spherical cavities,Im3m spacesymmetry

1D mesopores,p6mm hexagonal,

absence ofinterconnected

pores

Ia3d 3D cubiccontinuous pore

arrangement

Sponge-likeparticles,

warm-holemesostructured

framework

Pore diameter

Uniform andlarger pore

diameter (4–30 nm)facilitating easy

diffusion

Similar porediameter valuesbut nonuniform

mesopores

Smaller porediameter (1.5–10

nm) hindering thediffusion ofsubstrates

Smaller porediameter (2–3 nm)

hindering thediffusion ofsubstrates

Smaller porediameter thanSBA-15 (2–10)

Range of surfacearea

Higher surfacearea (~1000 m2/g),high surface areato volume ratio

Comparablesurface area values

to SBA-15

Lower surface area(~800 m2/g)

Higher surfacearea

(~1100 m2/g)

Surface area(800–1000 m2/g)

Stability

Thick walls (up to9 nm) and hencemore thermally

stable

Thick wallscomparable to

SBA-15

Thin walls (0.5 nm)and thus poorhydrothermal

stability

Thin walls andhence

comparatively lessthermally stable

Less orderedstructure butcomparable

stability

There are some important points that need to be considered both during the post-synthesis loading of the catalysts and in situ synthesis and loading of the catalyst particleson the porous silica-based supports. During in situ synthesis (generation), the catalystparticles get embedded inside the porous catalyst supports and may improve the overallmechanical stability of the porous support matrix. Instead, the post-synthesis loadingof the catalyst particles inside the porous supports may pose some problems such aspore blocking (as seen in some images in Figure 2). Therefore, the size of the catalystparticles during post-synthesis immobilization must be smaller than the pore size. Theblocking of the pores will prevent the reactant in various states to enter the pores and theoverall conversion will be low. The covalent immobilization of catalyst particles duringthe post-synthesis catalyst loading is also essential. The catalyst particles can be held bystrong bonds between the support and the surface of the catalyst. However, there is highlikelihood of compromising the catalyst surface for surface modification and subsequentcovalent immobilization reactions, which will affect the efficiency of the catalyst. The

J. Compos. Sci. 2021, 5, 75 5 of 17

percentage of loading (with respect to the weight of the porous catalyst support) of thecatalyst can greatly affect the stability of the support matrix as well as the catalytic efficiency.An optimum loading of the catalyst is necessary for efficient performance of the catalystin the reaction to be catalyzed. The shape of the pores available or created on the porousmatrix/particles also play an important role in determining the efficiency of the material. Ifthe shape of the catalyst particles and the pores is the same, then there is great possibility ofproper filling of the pores by catalyst particles. The post-synthesis covalent immobilizationof the catalyst and in situ synthesis and deposition of the catalyst on porous silica-basedsupports guarantee high stability of the material.

J. Compos. Sci. 2021, 5, 75 4 of 19

outstanding chemical and physical properties. The porosity plays an important role in

increasing the efficiency of catalyst supports. The shape as well as the size of pores of the

support have an important effect on the activity and stability of embedded catalysts. Metal

nanoparticles supported on porous silica-based supports exhibit higher catalytic activity

arising from the higher accessibility of the active sites. A variety of porous silica particles

are available as catalyst supports. Table 2 lists the main types of porous silica materials

used as catalyst supports and their characteristic properties. The chemical inertness and

high stability make these materials ideal catalyst supports. Figure 2 shows a representa-

tive example of how nickel nanoparticles are loaded or trapped inside different types of

mesoporous silica-based catalyst supports. The different arrangements of the embedded

catalyst inside the porous silica are clearly visible.

Figure 2. Schematic of the confinement effect on the Ni nanoparticles trapped inside the channels of supports. Reproduced

with permission from the authors of [37].

Table 2. Comparison of the properties of various porous silica particles.

Full

Name

Santa Barbara

Amorphous Type 15

Santa Barbara

Amorphous

Type 16

Mobil Composi-

tion of Matter

No. 41

Mobil Composi-

tion of Matter

No. 48

Hexagonal Meso-

porous Silica

Short

name SBA-15 SBA-16 MCM-41 MCM-48 HMS

Struc-

ture di-

recting

agent

Pluronic 123 (non-

ionic)

Pluronic F127

(non-ionic) CTAB (cationic) CTAB (cationic)

Amines (non-

ionic)

pH at

synthe-

sis

Acidic

(pH~1)

Acidic

(pH~1)

Basic

(pH~11–13)

Basic

(pH~11–13)

Basic

(pH~9)

Features Hexagonal pores, 2D

array, p6mm sym-

3D cubic arrange-

ment connected

1D mesopores,

p6mm hexagonal,

Ia3d 3D cubic

continuous pore

arrangement

Sponge-like parti-

cles, warm-hole

Figure 2. Schematic of the confinement effect on the Ni nanoparticles trapped inside the channels of supports. Reproducedwith permission from the authors of [37].

3. Types of Reactions

The versatility and stability of the porous silica-based catalyst supports are evidencedby a variety of chemical reactions catalyzed by different catalysts supported on them.The porous silica-based catalyst supports are used in the reactions such as aerobic ox-idation of alcohols (1-phenylethanol and benzyl alcohol) [38]; oxidation of methane tomethanol [39,40], propene [41], benzene, benzyl alcohol [42], and toluene [43]; oxidativeremoval of 4,6-dimethyldibenzothiophene [44]; photooxidation of CO [45]; CO oxidation atlow temperature [46–48]; organic oxidation of 1,2-dichloropropane [49,50]; photodegrada-tion of methylene blue [51]; CO2 adsorption [52–54]; hydrodeoxygenation of anisole [55];esterification to produce biofuels [56,57]; biodiesel production from palm acid distillate [58];biofuel upgrade (hydrocracking of camelina FAMEs) [59]; model transesterification reaction(ethyl acetate + methanol = methyl acetate) [60]; dehydration of glycerol [61]; hydrodesul-furization [62]; decomposition of N2O [63] and formic acid (HCOOH <-> H2 + CO2) [64];cycloisomerization of alkynoic acids to alkylidene lactones [65]; coupling reactions (cleansynthesis) [4,66,67]; carbon-carbon bond forming reactions [68]; removal of exhaust gaspollutant [69]; waste water treatment [70]; polymerization of olefins [71–73]; biodegrad-able polymer synthesis [74]; hydrocracking of pyrolyzed a-cellulose [75]; dehydrationtransfer hydrogenation [76]; selective hydrogenation of butadiene [77]; hydrogenation ofmethanol [78]; p-nitrophenol to p-aminophenol [79]; dehydrogenation [80–84]; CO2 tomethanol (CH3OH) [85]; epoxidation of styrene [86–88]; cyanosilylation [33]; reduction ofN2 to ammonia [89]; bromate [90]; photocatalytic water oxidation [91]; photocatalytic degra-dation and removal of Cr(VI) and methylene blue [92]; photocatalytic hydrogen evolutionfrom water [93]; catalytic transfer hydrogenation for the synthesis of Y-valerolactone [94,95];

J. Compos. Sci. 2021, 5, 75 6 of 17

Friedel-Craft alkylation reaction of aromatic compounds [96]; addition of carbon het-eroatom bond formation [97]; and soot combustion [98].

A classic example of use of porous silica as catalyst supports is shown in Figure 3. Itshows highly monodispersed palladium nanoparticles immobilized in three-dimensionaldendritic mesoporous silica used as catalyst in Suzuki-Miyaura C-C cross-coupling. Instead,Figure 4 shows AuPt alloy yolk@shell hollow nanoparticles and their incorporation intohollow interiors of a mesoporous silica microspheres based on a rapid aerosol process.The AuPt@SiO2 spheres showed excellent catalytic performance in the epoxidation ofstyrene with conversion and selectivity of 85% and 87%, respectively. Note that the variousreactions mentioned above include different reactions conditions and states of the reactants,solvents, and other chemicals, and the silica-based catalyst support withstands all thoseconditions, proving its chemical stability, which is the prime requirement of the catalystsupport as mentioned above.

J. Compos. Sci. 2021, 5, 75 6 of 19

tion (ethyl acetate + methanol = methyl acetate) [60]; dehydration of glycerol [61]; hy-

drodesulfurization [62]; decomposition of N2O [63] and formic acid (HCOOH <-> H2 +

CO2) [64]; cycloisomerization of alkynoic acids to alkylidene lactones [65]; coupling reac-

tions (clean synthesis) [4,66,67]; carbon-carbon bond forming reactions [68]; removal of

exhaust gas pollutant [69]; waste water treatment [70]; polymerization of olefins [71–73];

biodegradable polymer synthesis [74]; hydrocracking of pyrolyzed a-cellulose [75]; dehy-

dration transfer hydrogenation [76]; selective hydrogenation of butadiene [77]; hydro-

genation of methanol [78]; p-nitrophenol to p-aminophenol [79]; dehydrogenation [80–

84]; CO2 to methanol (CH3OH) [85]; epoxidation of styrene [86–88]; cyanosilylation [33];

reduction of N2 to ammonia [89]; bromate [90]; photocatalytic water oxidation [91]; pho-

tocatalytic degradation and removal of Cr(VI) and methylene blue [92]; photocatalytic hy-

drogen evolution from water [93]; catalytic transfer hydrogenation for the synthesis of Y-

valerolactone [94,95]; Friedel-Craft alkylation reaction of aromatic compounds [96]; addi-

tion of carbon heteroatom bond formation [97]; and soot combustion [98].

A classic example of use of porous silica as catalyst supports is shown in Figure 3. It

shows highly monodispersed palladium nanoparticles immobilized in three-dimensional

dendritic mesoporous silica used as catalyst in Suzuki-Miyaura C-C cross-coupling. In-

stead, Figure 4 shows AuPt alloy yolk@shell hollow nanoparticles and their incorporation

into hollow interiors of a mesoporous silica microspheres based on a rapid aerosol pro-

cess. The AuPt@SiO2 spheres showed excellent catalytic performance in the epoxidation

of styrene with conversion and selectivity of 85% and 87%, respectively. Note that the

various reactions mentioned above include different reactions conditions and states of the

reactants, solvents, and other chemicals, and the silica-based catalyst support withstands

all those conditions, proving its chemical stability, which is the prime requirement of the

catalyst support as mentioned above.

Figure 3. Synthesis and catalytic process of highly monodispersed palladium nanoparticles immobilized in three-dimen-

sional dendritic mesoporous silica. Reproduced with permission from in [99].

Figure 3. Synthesis and catalytic process of highly monodispersed palladium nanoparticles immobilized in three-dimensional dendritic mesoporous silica. Reproduced with permission from in [99].

J. Compos. Sci. 2021, 5, 75 7 of 19

Figure 4. AuPt nanoalloy yolk@shell hollow particles in ordered mesoporous silica microspheres. Reproduced with per-

mission from the authors of [100].

4. Recent Reports, Analysis and Trends

The very latest reports from the literature about porous silica-based catalyst supports

are enlisted in Table 3 with the name of the catalyst material and reactions catalyzed. Ob-

servation of the entries in Table 3 reveals some trends about the use of porous silica-based

materials as catalyst supports. Due to its simplicity and efficiency to produce monodis-

persed porous silica particles, the sol–gel technique remains the most common synthesis

technique to obtain the porous supports. The trend of in situ synthesis and loading of the

catalysts in the same synthesis conditions is also observed. The soft (easily degradable)

template approach is the preferred technique to generate the porosity, and it is observed

in most of the reports. Instead, a variety of different catalysts, such as noble metal nano-

particles, bimetallic nanoparticles, composite nanoparticles, alloys, noble metal, compo-

site material nanocrystals, etc., are embedded in the porous silica-based supports. The

pore sizes were tuned as per the sizes of various catalysts embedded in the materials. A

variety of new reactions are added to the previously reported reactions as pointed out

above. Most of the latest works also cover the studies of recyclability of the catalysts im-

mobilized on silica-based catalyst supports, this proves the good hydrothermal or sol-

vothermal (considering the fact that aqueous as well various solvents are used in the var-

ious reactions carried out) stability of the porous silica network. This is important from a

technology and cost point of view. All the reports suggest the improved catalytic perfor-

mance of the catalysts in various chemical reactions upon their immobilization on the po-

rous silica-based catalyst supports. In addition to mere catalyst supports, some of the very

interesting works report the chemical reactions catalyzed by porous silicas or by func-

tional (that is organic functional group bearing) porous silica nanoparticles.

Table 3. Summary of the latest reports from the literature about the use of porous silica-based catalyst supports.

Sr. No Material Type of Synthe-

sis Catalyst Chemical Reaction Ref.

1.

Core-shell struc-

tured magnetic

silica.

Sol-gel Bronsted acid

Transesterification of

soybean oils, low-

quality oils to bio-

diesel

[101]

2.

Chondroitinase

ABC (I) on red

porous silicon

nanoparticles

Electrochemical

etching chrondroitinase

Biological enzyme ca-

talysis reaction [102]

Figure 4. AuPt nanoalloy yolk@shell hollow particles in ordered mesoporous silica microspheres. Reproduced withpermission from the authors of [100].

J. Compos. Sci. 2021, 5, 75 7 of 17

4. Recent Reports, Analysis and Trends

The very latest reports from the literature about porous silica-based catalyst supportsare enlisted in Table 3 with the name of the catalyst material and reactions catalyzed.Observation of the entries in Table 3 reveals some trends about the use of porous silica-based materials as catalyst supports. Due to its simplicity and efficiency to producemonodispersed porous silica particles, the sol–gel technique remains the most commonsynthesis technique to obtain the porous supports. The trend of in situ synthesis andloading of the catalysts in the same synthesis conditions is also observed. The soft (easilydegradable) template approach is the preferred technique to generate the porosity, andit is observed in most of the reports. Instead, a variety of different catalysts, such asnoble metal nanoparticles, bimetallic nanoparticles, composite nanoparticles, alloys, noblemetal, composite material nanocrystals, etc., are embedded in the porous silica-basedsupports. The pore sizes were tuned as per the sizes of various catalysts embedded in thematerials. A variety of new reactions are added to the previously reported reactions aspointed out above. Most of the latest works also cover the studies of recyclability of thecatalysts immobilized on silica-based catalyst supports, this proves the good hydrothermalor solvothermal (considering the fact that aqueous as well various solvents are used inthe various reactions carried out) stability of the porous silica network. This is importantfrom a technology and cost point of view. All the reports suggest the improved catalyticperformance of the catalysts in various chemical reactions upon their immobilization onthe porous silica-based catalyst supports. In addition to mere catalyst supports, some ofthe very interesting works report the chemical reactions catalyzed by porous silicas or byfunctional (that is organic functional group bearing) porous silica nanoparticles.

Table 3. Summary of the latest reports from the literature about the use of porous silica-based catalyst supports.

Sr. No. Material Type of Synthesis Catalyst Chemical Reaction Ref.

1. Core-shell structuredmagnetic silica. Sol-gel Bronsted acid Transesterification of soybean oils,

low-quality oils to biodiesel [101]

2.Chondroitinase ABC

(I) on red poroussilicon nanoparticles

Electrochemicaletching chrondroitinase Biological enzyme catalysis

reaction [102]

3.Mesoporous-silica-supported metal

nanocatalystsSol–gel Metal nanocatalyst

(Ag, Pd, amines)

Dehydrogenation of formic acidfor Hydrogen generation(HCOOH -> H2 + CO2)

[103]

4. Mesoporous silica(SBA-15, MCM-41) Sol–gel

Palladium andplatinum

nanoparticles

Organic synthesis alcohols,carboxylic acids, and esters [104]

5.Mesoporous silica

spheres andnanocapsules

Soft and hard dualtemplate Sulfonic acid

Biomass valorization catalysis,conversion of cellobiose into

glucose[105]

6. Silica nanoshell(Pd/Fe3O4@h-SiO2) Sol–gel Pd nanocrystals

Biorthogonal Organic Synthesis,carbocyclization reactions,

converting a range ofnon-fluorescent substrates to

fluorescent products

[106]

7. Mesoporous fumedsilica Sol–gel Palladium, Cobalt,

Nickel, and Copper

Suzuki cross-coupling (SCC)reactions (C-C bond forming

reaction) Cross-coupling reactionbetween bromobenzene with

benzeneboronic acid givebiphenyl

[107]

J. Compos. Sci. 2021, 5, 75 8 of 17

Table 3. Cont.

Sr. No. Material Type of Synthesis Catalyst Chemical Reaction Ref.

8.MesoporousMoO3/SiO2

nanosphere networksSelf-assembly MoO3

oxidative desulfurization ofDibenzothiophene (DBT) [108]

9. Silica Sol–gel IrO2Photodegradation of methylene

blue [109]

10.Dendrimer-like Porous

Silica Nanoparticles(DPSNs)

Template-mediated

self-assemblyCu-BTC MOFs

Catalytic aerobic epoxidation ofolefins, cyclooctene to cyclooctane

oxide[110]

11. Mesoporous Silica(Fe@silica)

Sol-gel andHydrothermal Iron Oxidation [82]

12. Silica (CuO@SiO2) Sol-gel Cu nanoparticles(NPs)

Cu-catalyzed organictransformations, C-C bond

formation, reduction of organicdye

[111]

13. Mesoporous silicamatrix (MMS)

Direct growthtechnique

Hongkong Universityof Science and

Technology (HKUST-1)(Cu3(BTC)2, BTC 1

4benzene-1,3,5-tricarboxylate)nanoparticles

Condensation reaction,Friedlander reaction between

2-amino-5-chlorobenzophenoneand acetylacetone and Henry

reaction nitroaldol condensationbetween nitromethane and 4-

nitrobenzaldehyde

[112]

14.

Functionalizedmesoporous SBA-15,

SBA-16, MCM-41,MCM-48

Sol-gel Metal nanoparticles(Ti, V, Cr, and Mo)

Catalytic transformation, DryReforming of Methane (DRM)reaction as CH4 + CO2 -> 2H2+

2CO

[33]

15. Porous SiO2(Pt@HS-SiO2 PL) So-gel Platinum

Nanoparticles

Hydrogenation of alkenes (deceneto decane) and nitroarenes to

amino phenol[113]

16. Silica (SiO2) powder Sol-gel Diethylenetriamine Knoevenagel reaction(carbon-carbon (C-C) coupling). [114]

17. BiosilicaMicroparticles

Diatom(Thalassiosirapseudonana)

Horseradishperoxidase (HRP),

glucose oxidase, Goldnanoparticles

Biological enzyme catalysisreaction of T. pseudonana,

Oxidation of glucose.[115]

18.

Folicacid-functionalized

dendritic fibrousnano-silica

(FA-KCC-1-NH2)(FA = Folic acid,

KCC-I = fibrous nanosilica)

Hydrothermal KCC-1-NH-FAnanoparticles

Amidation of carboxylic acidswith amines [116]

19.

Monodispersemesoporous silica

microspheres(M-MSMs)

Sol–gel Au Nanoparticles Reduction of 4-nitrophenol (4-NP)to 4-amino phenol [117]

20. CaSiO3-SiO2 powder Co-precipitationmethod CaSiO3 (Na2O·nSiO2)

Decomposition of isopropylalcohol, dehydrogenation of the

alcohol producing acetone[118]

21. ReO3/SiO2, silicamatrix Sol-gel ReO3 nanoparticles Photodegradation of Blue

Methylene [119]

J. Compos. Sci. 2021, 5, 75 9 of 17

Table 3. Cont.

Sr. No. Material Type of Synthesis Catalyst Chemical Reaction Ref.

22.

Porous silicon dioxide(SiO2) and carboxyl-

functionalized carbonnanotube

(PtIrNi/SiO2-CNT-COOH)

Sol-gel PtIrNi alloynanoparticles

Electrochemical AmmoniaOxidation reaction (AOR) [120]

23.

Expanded mesoporoussilica

(EMSN)-encapsulatedPt nanoclusters.

Sol-gel Pt nanoclustersArtificial enzymes for tracking

hydrogen peroxide secretion fromlive cells

[121]

24. Microporous silicamicrocapsules

Gas-in-water-in oilemulsions(g/w/o)

Microporous silicamicrocapsules

Ostwald ripening, generation ofgas-in-water-in-oil emulsions [122]

25.Pt-loaded

ZSM-22/MCM-4(Pt-MES)

Sol-gel Bronsted acid, Ptnanoparticles

N-alkane isomerization forrefinery process by converting thepetroleum into the gasoline with

high quality and the diesel

[90]

26.

Organo-amine-functionalized castor

oil templatedmesoporous silicas

Valorization of ricehusk Amine groups

Biodiesel synthesis,transesterification of model

C4-C12 triglycerides (TAG) tofatty acid methyl esters

[123]

27.

TEMPO-functionalized

mesoporous silicaparticles, MCM-41 and

SBA-

Co-condensation

(2,2,6,6tetramethylpiperidin-

1-yl) oxyl(TEMPO)

Heterogeneous oxidation(oxidation of alcohols toaldehydes), Knoevenagelcondensation (C-C bond

formation)

[124]

28. Silica-encapsulatedcore–shell Co@SiO2

Hydrothermal Cobalt Fischer-Tropsch synthesis (FTS) [125]

29. Porous silica Sol-gel Gold nanoparticles Biomedical, catalytic, and opticalproperties [14]

30.

PalladiumNanocatalysts

Encapsulated onPorous Silica @

MagneticCarbon-Coated Cobalt

Nanoparticles

Sol-gel Palladiumnanoparticles

Sustainable hydrogenation ofnitroarenes to aniline, alkenes and

alkynes[126]

31.SBA-15-based

composites(X@SBA-15)

Impregnation andhydrothermal

methods

Transitionmetals/metal oxides

and nanocarbons

Water decontamination byadvanced oxidation processes [25]

32. Mesoporous silica Sol-gel Ni-Co bimetallichydroxide particles Urea oxidation reaction [127]

33.

Porous silicananotubes loaded Au

nanoparticles(SiO2@Au@SiO2 NTs)

Sol-gel Gold nanoparticles Catalytic reduction of4-Nitrophenol to 4-amino phenol [128]

34.Self-propelled

mesoporous silicananorods (MSNRs)

Sol-gel Iron oxide (Fe2O3)nanoparticles

Catalytic decomposition ofhydrogen peroxide by a sputtered

Pt layer[129]

35. Porous silica Self-assemblyMetal and alloy

nanoparticles (Au, Ag,pd, Ag/Pd)

Biomineralization, reduction of4-nitrophenol to 4-amino phenol [130]

J. Compos. Sci. 2021, 5, 75 10 of 17

Table 3. Cont.

Sr. No. Material Type of Synthesis Catalyst Chemical Reaction Ref.

36.

A novel andyolk/shell nanoreactorcatalyst (H-Fe3O4@h-

Cu0@m-SiO2)

Hydrothermal CuO-nanoparticles A3 coupling reaction of alkynes,aldehydes, and amines [131]

37. Micron-sized,spherical SiO2

Mechanochemical Spherical silica Polyolefin catalyst production [132]

38. Ni@SiO2 core–shellnanocatalysts Sol-gel Ni particles Catalytic oxidation of CH4 to CO2 [133]

39. Hollow SiO2 spheres Template synthesis Au nanoparticlesCatalytic Microreactors, reduction

of 4-nitrophenol to4-aminophenol

[134]

40.

MonolacunaryKeggin-type

[PW11O39] 7-(PW11)heteropolyanion

SBA-15(PW11@TMA-SBA-15)

Sol-gelN-

trimethylammonium(TMA)

Oxidative desulfurization oforganosilica composite [135]

41. Porous silica Sol-gel Ni nanocatalyst Thermal gasification of wastebiomass [136]

42. Macroporous SiO2 Sol-gel Ag2O, Na2O or K2O Soot combustion reactions,gas-solid-solid reactions [137]

43.

Aminopropylfunctionalized

mesocellular foamsilica (MCF)

Sol-gel Penicillin acylase6-aminopenicillanic acidproduction, biocatalytic

transformation[138]

44.TiO2/SiO2/C

nanofiber mat, SiO2nanoparticles

Calcination TiO2/SiO2/CPhotocatalytic degradation of

organic pollutants (rhodamine Band 4-nitrophenol) in water

[139]

45.Alumina-coated silica

nanoparticles(AlO-SiO NPs)

Sol-gel Alumina Surface reactions [140]

46. mesoporousg-C3N4/SiO2 material Sol-gel Carbon nitride

(g-C3N4)Photodegradation of rhodamine B

(RhB) under visible light [141]

47. Mesoporous silicamaterial KIT-6 Template synthesis Transition metals Electrocatalytic hydrogen

evolution reaction [142]

48. Pd/SiO2 and Fe/SiO2 Sol-gel

Metallic (Pd catalysts)or metallic oxide (Fe

catalysts)nanoparticles

Pd/SiO2 Hydrodechlorination of2,4,6-trichlorophenol (TCP) in

water, Fe/SiO2 materials degradephenol

[143]

49. Bimodal porous silica Sol-gel NiO Phenol to cyclohexanol [144]

50.Mesoporous silica

materials (SBA-15 andMCM-41)

Sol-gel Phosphonic andphosphoric acid esters

Asymmetric aldol reaction (C-Cbond formation) [145]

51. Mesoporous silica(SBA-15) Sol-gel Laccase Enzyme aggregate (E-CLEA)

potential in phenol removal [146]

52.

Novel hollow-Co3O4@Co3O4@SiO2 multi-

yolk-double-shellnanoreactors

Sol-gelMetals (Pd, Pt, Ru, Rh,

and Au) and metaloxides.

CO Oxidation [147]

J. Compos. Sci. 2021, 5, 75 11 of 17

Table 3. Cont.

Sr. No. Material Type of Synthesis Catalyst Chemical Reaction Ref.

53. Ordered mesoporoussilicas (MCM-41) Sol-gel Aluminum

Hydro isomerization andFriedel-Crafts alkylation of

benzene with benzyl alcohol[148]

54.Colloidal mesoporous

silica nanoparticles(LP-MSNs)

Co-condensation Alkyne-functionalized

Colorimetric reaction of guaiacol(2-methoxyphenol), hydrolysis of

4-nitrophenyl acetate (NPA) byLP-MSN-CA

[149]

5. Conclusions

This brief overview pointed out the importance of porous silica as catalyst supports.A huge number of reports in the literature on this topic prove the versatility and efficiencyof porous silica as catalyst supports. A careful observation of the latest reports showed thatsome previous trends about the synthesis of porous silica supports and in situ generationand immobilization of the catalysts are continued. A variety of new, bimetallic, compositeand functional nanocatalysts are embedded or immobilized on the porous silica nanoand microparticles to efficiently catalyze various reactions. The research in this field willproceed in future along following main directions.

Development of functional silica-based porous particles embedded with variouscatalyst nanoparticles where synergic effects of the various organic functional groupsgrafted on the supports and the catalyst will assist the chemical transformations.

Further development and optimization of single step or in situ (or minimum steps)methods for the functionalization of the silica-based porous supports as well as synthesisand immobilization of the catalysts.

Development of the large-scale production methods for already reported variouscomposite nanoparticles (catalyst) embedded in porous silica particles.

Author Contributions: Conceptualization, S.A.J.; writing—original draft preparation, P.S.S. (Preeti S.Shinde), P.S.S. (Pradnya S. Suryawanshi), K.K.P., V.M.B.; writing—review and editing, S.A.J., S.A.S.,S.D.D.; supervision, S.A.J., S.A.S., S.D.D.; project administration, S.A.J. All authors have read andagreed to the published version of the manuscript.

Funding: This research received no external funding.

Conflicts of Interest: The authors declare no conflict of interest.

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